There is a standard process for requesting parental leave, which is also set out in the Parental Leave Policy.
The employee can request a period of parental leave by writing to their manager (by email is fine), setting out the start and end dates for their proposed leave. The employee should give this notice at least 21 days before the start of the leave period. Where it's not possible to give 21 days' notice (e.g. if the child is born prematurely), the employee should let their manager know as soon as possible.